It has been conventionally considered that the capacitance of an electric double-layer capacitor is almost proportional to the surface area of polarizable electrodes (such as carbon electrodes) provided as an anode and a cathode constituting the electric double-layer capacitor. Therefore, in an attempt to increase capacitance by increasing the surface area of a carbon material of the carbon electrodes when the carbon electrodes are used as polarizable electrodes, various studies have been made.
For example, as a carbon material for a carbon electrode, activated carbon has so far been frequently used. Such activated carbon is generally produced by activating so-called hard-graphitization carbon, which is obtained by carbonizing coconut shell, wood chip, coal and phenol resin, with a gas such as water vapor or with a chemical agent such as an alkali metal hydroxide. However, the activated carbon produced from such hard-graphitization carbon has a problem in that the capacitance per unit volume does not increase proportionally with an increase of the specific surface area. To explain more specifically, in either one of activation reactions of hard-graphitized carbon with a gas or a chemical agent, fine pores are formed from the surface of a particle by an oxidative reaction. Therefore, as the activation reaction proceeds, the amount of loss due to oxidation increases. As a result, the activated carbon is low in bulk density. When such activated carbon is used in an electrode material, the bulk density of the resultant electrode decreases. Therefore, even if the capacitance per unit mass of the electrode increases, an increase of density of capacitance per unit volume of the electrode is limited. In addition, such activated carbon is low in electric conductivity due to hard-graphitization carbon. Because of this, the internal resistance of the electrode increases.
On the other hand, activated carbon obtained by activating easy-graphitization carbon has been proposed, which is obtained by infusibilizing/carbonizing mesophase pitch and a mesophase pitch carbon fiber obtained by spinning the mesophase pitch or by carbonizing petroleum coke and coal-tar pitch coke, etc., with an alkali metal hydroxide. When the easy-graphitization carbon is activated with an alkali metal hydroxide, activated carbon is produced in high yield and with high bulk density. Therefore, an electrode having a high bulk density can be formed. Furthermore, the density of capacitance per unit volume can be increased. Moreover, the activated carbon produced from the easy-graphitization carbon generally has a high electrical conductivity compared to that produced from hard-graphitization carbon. Therefore, the activated carbon produced from the easy-graphitization carbon has also an advantage of easily reducing the internal resistance of an electrode.
However, the mesophase pitch and a mesophase pitch carbon fiber obtained by spinning the mesophase pitch are expensive materials themselves. In addition to this problem, there is another problem in that since an infusibilizing/carbonizing process must be performed prior to an activation process, manufacturing steps becomes complicated, raising manufacturing cost more and more. A still another problem resides in that since oxygen is introduced by an infusibilizing reaction, the degree of graphitization decreases, with the result that the internal resistance of an electrode does not decrease so much.
On the other hand, JP-A-10-199767 (Patent Document 1) proposes a method of carbonizing petroleum coke or coal pitch coke to obtain a product satisfying conditions: a volatile content of 1.0 to 5.0 wt. % and an H/C atom ratio of 0.05 to 0.30, followed by activating the product with an alkali metal hydroxide.
Furthermore, JP-A-2003-51430 (Patent Document 2) describes that a raw-material carbon composition, which has an HGI (Hardgrove Grindability Index, defined by ASTMD-409-71) of 50 or more, a microcrystalline-carbon interlayer distance: d002 of 0.343 nm or less, and a lattice constant of crystal: Lc002 of 3.0 nm or less, is activated with an alkali, and that a carbon material for an electrode of an electric double-layer capacitor having a large capacitance per unit volume can be obtained.
Patent Document 1: JP-A-10-199767
Patent Document 2: JP-A-2003-51430